1. Field of Invention
The present invention relates to a bridge inverter ballast for a fluorescent lamp having thermionic cathodes that need to be pre-heated before the lamp is properly ready to be ignited.
2. Prior Art
A typical fluorescent lamp have thermionic cathodes that need to be brought to incandescence before the lamp is properly operable be ignited--that is, ready to receive its main operating power.
When, as frequently is desirable to do, such a lamp is powered by way of being parallel-connected with the tank capacitor of a high-Q resonant circuit that is series-excited by the voltage output of an inverter power supply, a problem arises: before the lamp is properly operative to absorb power, the high-Q resonant circuit is effectively unloaded and it therefore represents an effective short circuit across the inverter's voltage output. This not only causes an excessively large power drain from the inverter, but also causes excessively large-magnitude voltages to develop across the circuit elements of the resonant circuit.
One way of circumventing this problem is that of providing the requisite cathode heating power from a separate source of power, and to delay the turning-on of the inverter power supply until after the cathodes have reached incandescence. However, this approach requires the use of an extra power supply, and is therefore not as cost-effective as might be desired.